The following mutants of Chinese hamster cells in culture have been isolated to date: 1. Purine requiring auxotrophs. 2. Resistance to 8-azaguanine and 2, 6 diamino-purine (HGPRT- and APRT-). 3. Methylmercapto purine ribonucleoside resistance (adenine kinase-). 4. Alpha-amanitin resistance. All of these mutants are being used in studies of macromolecular regulation in mammalian cells. In particular the relationship between mutation of the purine salvage enzymes and the rate of purine biosynthesis by the de novo pathway is being studied. Comparative studies are being carried out using human cells derived from individuals with Lesch-Nyhan Syndrome. The effect of purine deprivation on mammalian cells is being studied utilizing purine auxotrophs. Despite the absence of purines, such auxotrophs remain viable for 7-10 days after being placed in starvation media. These cells replenish their nucleotide pools by breakdown of ribosomal RNA. Fluctuations in the nucleotide pool are being studied by thin-layer chromatography.